The enzyme, isolated from Ehrlich tumor cells, catalyzes the incorporation of CMP from the substrate CTP into a RNA primer molecule. ATP, while a substrate, is a much poorer substrate than CTP. While the enzyme is highly purified (1700-fold) it is not homogeneous. In the proposed study we will purify the enzyme to homogeneity and study the physical and chemical properties of the isolated protein (e.g. subunit structure, amino acid composition). The RNA which serves as primer is quite specific in that all RNA's do not serve as primer. The most active primer is an RNA fraction isolated from the tumor cells; yeast sRNA is only 1/50 as active as the tumor cell RNA. The nature of the RNA specificity is to be studied to determine if the base composition of the RNA or the 3'-OH terminus is the more critical factor in determining primer activity. In addition, studies will be carried out to determine the in vivo function of the enzyme.